A marine seismic streamer is an elongate cable-like structure, typically up to several thousand meters long, which contains arrays of hydrophones and associated electronic equipment along its length, and which is used in marine seismic surveying. In order to perform a 3D (as well as 2D or 4D) marine seismic survey, a plurality of such streamers is towed at about 5 knots behind a seismic survey vessel, which also tows one or more seismic sources, typically air guns. Acoustic signals produced by the seismic sources are directed down through the water into the seabed, where they are reflected. The reflected signals are received by the hydrophones, and then digitized and processed to build up a representation of the area being surveyed.
The streamers are typically towed at a constant angle relatively to the ship, in order to facilitate the removal of undesired “ghost” reflections from the surface of the water. To keep the streamers at this constant depth, control devices known as “birds”, attached to each streamer at intervals of 100 to 300 meters, are used.
Current designs of birds comprise a relatively heavy body which is suspended beneath the streamer, and which has a pair of laterally projecting wings (hence the name “bird”), one on each side. The combination of streamer and birds is arranged to be neutrally buoyant, and the angle of attack of both wings is adjusted from time to time to control the depth of the streamer.
During a seismic survey, the streamers are intended to remain straight parallel to each other and equally spaced. However, after deploying the streamers, it is typically necessary for the vessel to cruise in a straight line for at least three streamer lengths before the streamer distribution approximates this ideal arrangement and the survey can begin. This increases the time taken to carry out the survey, and therefore increases the cost of the survey. However, because of sea currents, the streamers frequently fail to accurately follow the path of the seismic survey vessel, sometimes deviating from this path. This can adversely affect the coverage of the survey, frequently requiring that certain parts of the survey be repeated.
U.S. Pat. No. 5,443,027 describes a lateral force device for displacing a towed underwater acoustic cable, providing displacement in the horizontal and vertical directions, the device having a spool and a rotationally mounted winged fuselage.
U.S. Pat. No. 6,011,752 describes a seismic streamer position control having a body with a first end and a second end and a bore therethrough from the first end to the second end for receiving a seismic streamer therethrough, at least one control surface, the at least one recess in which is initially disposed the at least one control surface movably connected to the body for movement from and into the at least one recess and for movement, when extended from the body, for attitude adjustment.
U.S. Pat. No. 6,144,342 describes a method for controlling the navigation of a towed linear acoustic antenna. Birds equipped with variable-incidence wings are fixed onto the antenna. Through a differential action, the wings allow the birds to be turned about the longitudinal axis of the antenna so that a hydrodynamic force oriented in any given direction about the longitudinal axis of the antenna is obtained. The bird can be detached automatically as the antenna is raised so that the antenna can be wound freely onto a drum. This method also allows full control of the deformation, immersion and heading of the antenna.
U.S. Pat. No. 6,671,223 describes a control device or “bird” for controlling the position of a marine seismic streamer, which is provided with an elongate, partly flexible body which is designed to be electrically and mechanical connected in series with a streamer. In a preferred form the bird has two opposed wings which are independently controllable in order to control the streamers lateral position as well as depth.
Birds in accordance with these current designs suffer from a number of disadvantages. Because the birds (in some of the patents mentioned above) hang beneath the streamer, they produce considerable noise as they are towed through the water. This noise interferes with the reflected signals detected by the hydrophones in the streamers. Some of the birds comprise a pair of wings or rudders mounted on a rotatable structure surrounding the seismic steamer in order to generate lift force in a specified direction. This is an expensive and relatively complex electro-mechanical construction that is highly vulnerable in underwater operations. This also adds complexity to the local control loop, and also slows down the response time of the overall streamer positioning control system.
There is a need to provide a bird, which facilitates the combination of precise positioning of the streamer spread and the ability to steer individual streamers both in shape and position relative to other streamers. There is also a need to provide a control device which allows faster course change operations and shorter run-ins after line-turns than current designs.